A Long Term Evolution (LTE) system is a communication system which adopts an Orthogonal Frequency Division Multiplexing (OFDM) multi-carrier modulation technique in the physical layer. The fundamental principle of the OFDM technique is to convert a high-speed series data stream into N low-speed data streams for parallel transmission. These data streams are carried by orthogonally separable sub-carriers and can effectively combat frequency-selective fading and improve system throughput significantly. However, for the OFDM technique, a timing deviation or a frequency deviation may extremely degrade the system performance. Thus, timing synchronization and frequency synchronization are theoretical and practical problems that have to be solved in the LTE system.
The timing synchronization can be categorized into wireless data frame timing synchronization, OFDM symbol block synchronization and sampling clock synchronization. The wireless data frame timing synchronization and the OFDM symbol block synchronization are mainly discussed herein. An OFDM symbol block consists of a Cyclic Prefix (CP) and a payload. The OFDM symbol block synchronization is to determine the start time of the payload in an OFDM symbol. Here, a timing offset will result in a phase rotation of a sub-carrier. This can be interpreted using the property of Fourier transformation that an offset in time domain corresponds to a phase rotation in frequency domain. If an addition of a timing offset and a length of maximum delay spread of channel is smaller than the length of the CP, orthogonality among the sub-carriers may be maintained and there is no Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI). However, if the timing offset and the length of maximum delay spread of channel is larger than the length of the CP, the orthogonality among the sub-carriers will be damaged, resulting in ISI and ICI that significantly degrades the system performance.
On the other hand, a carrier frequency offset, or frequency offset for short, is mainly caused by a difference in local carrier frequencies between a transmitter and a receiver, a Doppler shift and the like. An integral carrier frequency offset will not cause any ICI, but will result in a 50% of error probability for information symbols demodulated at the receiver. A fractional carrier frequency offset will damage the orthogonality among the sub-carriers and thus cause ICI. In order to achieve a carrier frequency synchronization between the transmitter and the receiver, it is necessary to estimate and then compensate for the frequency offset.
During the implementation of the present invention, the inventors have found at least the following problem. The LTE-related protocols only define various specifications for the uplink transmitter, but do not define any specific implementation for the downlink receiver. Hence, with the development of the synchronization techniques, a number of different schemes for timing synchronization and frequency offset estimation have been proposed. Most of the existing schemes for timing synchronization and frequency offset estimation, despite their high synchronization accuracy, are time-consuming due to a very high computation load associated with these schemes for timing synchronization and frequency offset estimation.